Solution and Solid State Characterization of a Cadmium Octaazacryptand Complex

Abstract

The constant for cadmium binding by octaazacryptand L = N(CH2CH2NHCH2CH2NHCH2CH2)3N was determined by potentiometry; log βML =18.3(1). Calculation of pM values (pM = −log [M]) show that L has the highest reported binding affinity for cadmium relative to other [2.2.2] polyoxa−polyaza cryptates. Two cadmium cryptate complexes, [CdL](BF4)2 (1a) and [CdL](OAc)2·2H2O (1b), were synthesized by metalation of the free cryptand. The molecular structures of compounds 1a and 1b were determined by X-ray diffraction. Crystallographic data for 1a: trigonal, P3̄1c, Z = 2, a = 9.581(2) Å, c = 17.003(4) Å, V = 1351.2(5) Å3. Crystallographic data for 1b: monoclinic, C2/c, Z = 4, a = 10.500(2) Å, b = 14.782(3) Å, c = 19.395(4) Å, β = 99.30(2)°, V = 2971(1) Å3. The cadmium ions in both complexes are eight-coordinate and display similar coordination geometries but different ligand conformations, designated as parallel and oblique. The cadmium coordination sphere is best described as a bicapped octahedron. Through 2D NMR experiments we have found that the Cd is in a symmetric environment and the conformation of the ligand in solution is consistent with the parallel conformation observed in one of the solid state structures. The 113Cd NMR shift of this eight-coordinate amine complex is 75 ppm relative to 0.1 M aqueous Cd(ClO4)2, which correlates with shifts observed for other [2.2.2] cadmium polyoxa−polyaza cryptates.